1. Field of the Invention
This invention relates to curing agents for coatings. More particularly this invention relates to the use of certain N,N'-dialkylalkylenediamines as curing agents for blocked isocyanate coatings. The coatings formulated with the products of this invention have better storage stability than those which utilize other curing agents. In addition, heat cured films made from these coatings have good properties.
2. Related Art
It is known in the art to combine various compounds with isocyanates to provide coating compositions with a variety of desirable properties.
A non-aqueous poly(urethane-urea) is disclosed in U.S. Pat. No. 4,590,254. It is prepared by reverse-addition of an isocyanate-terminated prepolymer to an organic medium containing an alcohol solvent and a chain extender comprising at least two isocyanate-reactive active hydrogen groups.
In U.S. Pat. No. 4,490,520 there is disclosed a process for preparation of an impact-resistant polyamide which comprises polymerizing an .omega.-lactam in contact with (1) an alkalai catalyst and (2) the reaction product of (a) a polyfunctional cocatalyst and a polyoxyalkyleneamine. The polyamides of this process are particularly useful for making strong tough molded or cast articles.
Wolf et al. disclose in U.S. Pat. No. 4,495,229 a polyurethane one-component heat-cured coating which is stable in storage, may be hardened above 120.degree. C. and is made of a polyol compound having a low glass transition temperature and a blocked polyisocyanate, where the blocked isocyanate is the reaction product of a polyisocyanate compound and a secondary amine.
Novel difunctional isocyanate prepolymers have been synthesized from polyoxyalkylenediamines and diisocyanates, as exemplified in U.S. Pat. No. 4,761,465. The novel products contain two active isocyanate groups linked by at least two urea and polyoxyalkylene moieties.
It is known that some agents reduce side reactions in isocyanates during storage. In U.S. Pat. No. 4,590,009 there is disclosed a method of reducing side reactions during the preparation and storage of a compound having a free isocyanate group by the addition of 50 to 5000 ppm hydrazine hydrate based on the amine reacted with the polyisocyanate group.
A very good overview of blocked isocyanates can be found in a Mobay publication titled "Blocked Isocyanates in Coatings", Mobay Chemical Corp., presented at Water-Borne and Higher-Solids Conference, Feb. 5-7, 1986, New Orleans, La. Many aspects of isocyanate chemistry are discussed including background, properties, raw materials, advantages of blocked polyisocyantaes, blocking agents, etc.
A partially alkoxylated polyoxyalkyleneamine has been used as a curing agent. In U.S. Pat. No. 4,767,836 a storage stable polyurethane coating composition is disclosed which may be heat cured under relatively mild conditions. It comprises an isocyanate which has been chemically blocked and a curing agent comprising a partially alkoxylated polyoxyalkylene amine.
In a U. K. Patent to USM Corporation (U. K. 2,151,643) there is disclosed a heat curable composition comprising (a) polyurethane prepolymer formed by reacting a polyisocyanate with a polyol, (b) a blocking agent and (c) a curing agent comprising an amine tipped polyoxypropylene polyether triol.
A one pot or package polyurethane system having the advantage of easy handling is disclosed in U.S. Pat. No. 4,624,996. The system comprises an isocyanate-terminated polyurethane prepolymer the isocyanate groups of which are blocked by reaction with an oxime, and a curing agent having at least two active hydrogen atoms per molecule.
In related copending case U.S. application Ser. No. 078,309 a series of novel diamines was prepared from the reaction of polyoxyalkylenediamines and diisocyanates.
Much information is available on the properties of various amines in a product booklet titled, "Amines", Virginia Chemical Company, Portsmouth, Va.
It is always useful in the art to identify compounds useful as curing agents for coatings, especially those which will provide products with improved storage stability.
It is known that N,N'-di-tert-butylethylenediamine can be used as an epoxy hardener. Technical Data, Bulletin 1100.95, Virginia Chemical Company. However, in the instant invention it was found that said amine is not suitable for use as a curing agent for blocked isocyanates. Now it has surprisingly been discovered that certain N,N'-dialkylalkylenediamines having a definable structure are useful curatives for blocked isocyanates. N,N'-diisopropylethylenediamine and N,N'-diisobutylethylene-diamine are among the definable alkylenediamines which provide cured blocked isocyanate products that are more stable than products resulting from blocked isocyanates cured with other amines.